1. Field of the Invention
The present invention relates to an air/fuel ratio control system for an internal combustion engine having an adaptive controller which calculates a feedback correction coefficient to correct the amount of fuel supplied to the engine such that the detected air/fuel ratio converges to a desired air/fuel ratio.
2. Description of the Related Art
Conventionally, it is known to install an air/fuel ratio sensor in an exhaust system of an internal combustion engine having a catalyst (catalytic converter) which is provided at the engine exhaust. The air/fuel ratio sensor detects the air/fuel ratio of the engine exhaust and is used to control the amount of fuel supplied to the engine (the fuel injection amount) in a closed-loop manner such that a detected air/fuel ratio converges to a stoichiometric air/fuel ratio, since the purification efficiency of the catalyst becomes maximum at or around the stoichiometric air/fuel ratio.
The assignee proposed in Japanese Laid-Open Patent Application No. Hei 7(1995)-247886 the above described control system using an adaptive controller in which a basic amount of fuel (basic fuel injection amount) was calculated a) by retrieving 20 mapped data, prepared beforehand, using engine parameters indicative of engine operation such as engine speed, and b) by correcting the basic amount of fuel by using a feedback correction coefficient calculated by the adaptive controller. Specifically, the adaptive controller was configured to receive the detected air/fuel ratio (referred to as KACT) as the controlled variable y and the desired air/fuel ratio (referred to as KCMD) as the desired value r, and to calculate the feedback correction coefficient (referred to as KSTR) as an output u such that the detected air/fuel ratio KACT becomes equal to a desired air/fuel ratio KCMD.
More specifically, as illustrated in FIG. 14, the adaptive controller was defined by a control law expressed in a recursion or recurrence formula and had an adaptation mechanism that estimates adaptive parameters (system parameters or controller parameters) .theta.(k) expressed as a vector (transpose vector) which estimated or identified the dynamic characteristics of the controlled object or plant (the engine). Based on the estimated adaptive parameters .theta.(k), the adaptive controller calculated the feedback correction coefficient KSTR(k). As shown, the adaptive parameters .theta. comprises elements of b0, r1, r2, r3 and s0 if the dead time (or delay time) d of the system is assumed to be 3 (3rd order).
Moreover, it has been proposed in Japanese Laid-Open Patent Application No. Hei 3 (1991)-185244 to install both a wide-range air/fuel ratio sensor (sometimes referred to as a universal sensor that detects the air/fuel ratio ranging from lean to rich) at a location upstream of the catalyst and an O.sub.2 sensor at a location downstream of the catalyst in the engine exhaust, and to correct or determine the desired air/fuel ratio within a range (the so-called catalyst window) in response to the output of the O.sub.2 sensor, thereby maximizing the catalyst purification efficiency. The amount of fuel supplied to the engine was then controlled in response to the corrected desired air/fuel ratio and the output of the wide-range air/fuel ratio sensor. In the technique disclosed, the controlled object is modeled and an optimum regulator is designed for controlling the amount of fuel supplied to the engine.
With regard to the above-mentioned air/fuel ratio control system previously proposed by the assignee, the inventors tested the response of the detected air/fuel ratio KACT((k) on an actual engine when the desired air/fuel ratio KCMD(k-d') was stepwise changed, while keeping the engine parameters unchanged (i.e., at the engine speed of 2800 rpm under a negative manifold pressure of -150 mmHg). Contrary to what was expected, it was found that the detected air/fuel ratio KACT(k) did overshoot the desired air/fuel ratio KCMD(k-d'), as illustrated in FIG. 15(a). FIGS. 15(b) to 15 (f) show the response of the elements of the adaptive parameters .theta. at that time.
Here, "k" means a sampling number in a discrete-time series, and more precisely an air/fuel ratio control cycle, and "KCMD(k-d')" means the desired air/fuel ratio corresponding to the detected air/fuel ratio KACT(k) but earlier from the time k by the dead time d.sup.1. The dead time is generally expressed as d and is specifically expressed as d'. It should be noted here that, to simplify calculation, the desired air/fuel ratio and the detected air/fuel ratio are both expressed, throughout the description, as the equivalence ratio, i.e., as EQU Mst/M=1/.lambda.
where, Mst is a stoichiometric air/fuel ratio; M is A/F (A is an air mass flow rate; F is a fuel mass flow rate); and .lambda. an excess air factor. In the test results shown in FIG. 15(a), the desired air/fuel ratio was varied 0.98 to 1.02, and centered at 1.0 in terms of the equivalence ratio.
As is clear from the figures, all elements except for b0 varied with respect to time. Since the desired air/fuel ratio KCMD was used as the desired value r in the calculation of the adaptive parameters, the change of the desired air/fuel ratio caused the adaptive parameters to fluctuate.
Thus, when the desired value input to the adaptive controller was changed frequently, this acted as a disturbance, causing the calculated adaptive parameters to fluctuate. Since the adaptive parameters are used in calculating the feedback correction coefficient, the calculated feedback correction coefficient would not be free from the adaptive parameters fluctuation. This would degrade air/fuel ratio control and lower the adaptive controller stability.
The above is an example. The problem will similarly happen when the desired air/fuel ratio is changed or perturbed in response to the operating conditions of the vehicle (such as the engine load, the engine speed and engine coolant temperature, etc.), or when the desired air/fuel ratio is repeatedly corrected within the catalyst window in response to the output of the O.sub.2 sensor installed downstream of the catalyst so as to maximize the catalyst purification efficiency, as proposed in the above reference (3-185244).